1. Field of the Invention
The invention relates to a process for removing AlCl3 from organochlorosilane mixtures.
2. Description of the Related Art
In the preparation of organochlorosilanes, product streams are obtained which may comprise aluminium chloride in different concentrations as a function of the process/operation. Depending on the conditions, for example the temperature and the chemistry of the organosilanes, the AlCl3 is dissolved at least partly in the liquid organochlorosilane stream and thus cannot be removed by filtration. Being a Lewis acid, AlCl3 can exert highly disruptive influences depending on the process temperature.
An effective distillative removal of organochlorosilanes and AlCl3 is possible only at moderate temperatures up to about 150° C., since AlCl3 exhibits the tendency to sublime and at least partly distils overhead with the organochlorosilanes when higher temperatures are employed. According to Ullmann's Encyclopaedia of Industrial Chemistry, AlCl3 has a sublimation temperature of 181.2° C. at 101.3 kPa.
According to Ullmann's Encyclopaedia of Industrial Chemistry, AlCl3 has a sublimation temperature of 181.2° C. at 101.3 kPa.
Disruptive Influences of AlCl3 are, for example:
1. At relatively high temperatures, AlCl3 reacts with siloxanes, for example methylchlorodisiloxanes, to form aluminosiloxanes. Depending on the degree of branching of these aluminosiloxanes, they are viscous to solid and can thus clog process equipment such as pipelines, or greatly reduce heat transfer as a result of deposits in the region of heated process equipment such as heat exchangers. The formation of such aluminosiloxanes is described, for example, in “W. Noll, Chemistry and Technology of Silicones, 1968, pages 238 and 340-342 (1).
2. AlCl3, even at low temperatures, is an excellent catalyst for the exchange of the organic ligands and the Cl and H ligands in organochlorosilanes, especially in the presence of Si—H compounds. These reactions are sometimes used selectively for the preparation of certain organochlorosilanes. However, this ligand exchange may also be disruptive when the AlCl3 has not been added deliberately and the product spectrum is thus shifted in an undesired direction. Such ligand exchange reactions are described, for example, in (1), pages 57-66 and J. Organomet. Chem. 260 (1984), 31-39, H. Schmölzer, E. Hengge (2).
FR 2761360 states that, for example, the selective addition of compounds of the (R)—Si—(OR) type can reduce the catalytic effectiveness of AlCl3 in relation to ligand exchange. However, this method has the following disadvantages: an additional substance has to be used, which increases cost, and subsequently has to be destroyed/disposed of again, and which complicates the distillative workup of the organochlorosilanes.
3. AlCl3 catalyses the decomposition of methylchlorodisilanes in the direction of methylchloromonosilanes and oligo-/polysilanes; in the presence of Si—H bonds, this reaction begins at as low as from approx. 105° C. The oligo-/polysilanes formed may be viscous to solid and insoluble, and may induce the problems described under point 1. The formation of these oligo-/polysilanes is described, for example, in (2).
The presence of AlCl3 is particularly disruptive in the processes below:
1. Direct Synthesis of Methylchlorosilanes According to Müller-Rochow.
In the direct synthesis of methylchlorosilanes, silicon is reacted with MeCl in the presence of various catalysts at about 265-310° C. This forms a mixture of various methylchloro(hydro)silanes, and also methylchlorodisilanes, methylchlorodisiloxanes and hydrocarbons. The Si used typically contains 0.1-0.3% Al, and an increase in the amount of Al, for example by adding an aluminum-containing alloy to the reaction system is likewise known. Irrespective of the source and of the form used, AlCl3 forms at least partly from the aluminium and, owing to the temperatures, the system comprising the reaction products and unconverted starting materials leaves via the gas phase.
In “Catalysed Direct Reactions of Silicon; K.M. Lewis, D.G. Rethwisch; Elsevier 1993; Chapter 1” (3), FIG. 3 on page 18 shows a schematic flow diagram of the process, in which the solid-containing reaction products from the direct synthesis are condensed, the solids are removed and the crude silane is fed to the distillation. “Ullmann's Encyclopaedia of Industrial Chemistry Vol. A 24, page 26” describes a similar process.
The liquid crude silane mixture prepared in this way comprises, in addition to the methylchloromonosilanes, also AlCl3, methylchlorodisilanes, disiloxanes and hydrocarbons. This means that the reactions described under “disruptive influences” occur during the distillative workup even when only the filtered crude silane mixture freed of solid is used further.
In (3) on page 22-28, the following further workup method is specified as an alternative:
“Gases separated in the cyclone and filter are fed into the bottom of a scrubber in which products with normal boiling points less than about 170° C. are separated from metal chlorides and other higher boilers. The distillate is fractionated into an overhead stream containing compounds boiling at or below 71° C. and a side stream composed essentially of the cleavable disilanes. The bottoms, containing solids and methylchlorosilanes, are purged periodically and sent to waste disposal.”
The disadvantages in this process are: Since the methylchlorosilanes are a mixture of many different substances having a wide boiling point range, it is not possible simultaneously to drive all utilizable products out of the bottoms of the scrubber and/or of the fractionation unit and to keep the temperature for the driving-out of the organochlorosilanes so low that the disadvantages described do not occur. In other words, the scrubber or the fractionation unit is operated at temperatures at which the reactions catalysed by AlCl3 do not occur to a noticeable extent, and the loss of utilizable methylchloro(di)silanes is automatically accepted. However, when these plants are operated at a higher temperature at which almost all utilizable products are driven out, the undesired side reactions occur to an increased extent, and the higher-boiling fractions, for example the disilane fraction, simultaneously comprise not inconsiderable proportions of entrained AlCl3. The residues which occur are suspensions composed of liquid organochlorosilanes and solids. A workup or disposal of such product streams is generally to be classified as problematic.
When FIGS. 4 and 5, page 25, 26 and illustrative text in (3) are considered, it can be seen that the AlCl3 introduced with the crude MCS direct reaction mixture is discharged with the disilanes in the direction of column A and will cause the problems already described many times, in this region or in the downstream disilane workup at the latest.
2. AlCl3-catalysed High Boiler Workup.
EP 829484 A, for example, describes the AlCl3-catalysed cleavage of high boilers from methylchlorosilane synthesis by means of HCl or H2 or corresponding mixtures. EP 155626 A, for example, describes the AlCl3-catalysed conversion of high boilers and low boilers in the direction of more utilizable monosilanes. In the workup of these reaction products, comparable problems occur to those which have been described for the direct synthesis.
3. Amine-catalysed Disilane Cleavage.
Various methylchlorodisilanes which are obtained as a by-product in direct synthesis may be converted using hydrogen chloride directly to methylchloromonosilanes (disilane cleavage). This reaction is catalysed, for example, by tertiary amines such as tributylamine, and is described in (3) on page 30-31. However, AlCl3 forms complexes with amines which have only a greatly reduced, if any, catalytic activity, i.e. when AlCl3 is present in sufficient amounts in the disilane cleavage, the reaction comes to a standstill.